This ESTCP project evaluated monitored natural attenuation (MNA) of explosives in groundwater at the Louisiana Army Ammunition Plant (LAAP), a former explosive-waste disposal lagoon area where the source material had been removed. The objectives were to (1) determine whether MNA of 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitro-1,3,5-hexahydrotriazine (RDX) could be confirmed through effective monitoring and (2) optimize MNA data collection and processing procedures. MNA relies upon natural biotic and abiotic processes to reduce the amount of contaminants in groundwater to acceptable levels.

Thirty LAAP groundwater monitoring wells were sampled quarterly over a two year period to document decreasing contaminant concentrations. Groundwater samples were analyzed for relevant inorganic parameters, as well as for TNT, RDX, their by-products, and their degradation constituents (EPA Method 8330). A cone penetrometer study was performed to gather site hydrogeology data and to collect aquifer material for bioassays which included radioassays, lipid biomarker analyses, and nucleic acid analyses. These analyses were used to develop techniques to follow the attenuation processes and estimate process rates. The two years of groundwater monitoring data, the numerical groundwater models, and the attenuation process rates predict a reduction in contaminant mass over the next 20 years.

Site Heterogeneity illustrated by three-dimensional mesh system used in FEMWATER (colors representing different materials or hydraulic conductivity distribution) The original and average

MNA is projected to cost 25 percent less than in-situ bioremediation and 50 percent less than 20 years of groundwater pump and treat operations. The cost savings increase significantly if the treatment duration extends beyond 20 years. MNA is an attractive alternative for meeting remedial goals where disturbance of site improvements or sensitive habitat is undesirable or where engineered remedial alternatives are not feasible.

MNA can take a long period of time to meet the cleanup goals and can require institutional controls to protect potential receptors until the goals are met. MNA requires a significant amount of data to confirm that it is a viable remedial alternative. Most of the bioassay techniques are non-standard analyses that are still developmental. MNA acceptance by the EPA and the affected community is dependent on the quality of the data generated and on whether the processes are predicted to be sufficiently fast to protect receptors from contamination. (Project Completed - 1999)